II. The Formation of Sulfides

contain large amounts of sulfides, and oxidation of these introduces serious

problems in some lignite, bituminous, anthracite, pyrite, copper, zinc, and

lead mines (Temple and Koehler, 1954). Oxidation of sulfides may provide sulfates for charging ground water in lower-lying areas and these, suggests Poelman (1973b), are the source of sulfur for further pyrite formation in these waterlogged areas. Subsequent drainage and oxidation Iead

to sulfate formation and the creation of “cat sands,” sandy soils with

jarosite mottles. Inland swamps at 2000 m have been described by Chenery

(1953, 1954) in Uganda where the sulfur comes from surrounding formations. Sulfides weather to sulfates and encrustations of sodium sulfate appear in the nearby area, providing salt licks and soluble salts that enter

the swamps in drainage water. Sulfides formed by reduction accumulate

in these swamps. A similar situation has been described by Thompson

(1972) in Rhodesia where the sulfates are thought to originate from deep

seated springs and are then reduced in the peaty swamps in some areas.

Solfataras may also provide excess sulfur in surrounding soils leading to

high acidity.

Sulfur from biological materials, algae, diatoms, etc., is described by

Sombatpanit (1970) as the source of sulfides in some “gyttja” soils in

Sweden. These soils are formed by the simultaneous sedimentation of fine

mineral particles and plant and animal remains in rivers and lakes. Other

gyttja deposits in Sweden and Finland originate in marine or brackish

environments.

B. SULFATE REDUCTION

IN ANAEROBIC

SOILS

When Hamlet asked, “How long will a man lie i’ th’ earth ere he rot?’

he was told, “. . . some eight year. A tanner will last you nine year.”

Shakespeare may have underestimated the effect of tannins on protein, as

the preservation of Iron Age bodies for 2000 years in Danish peat bogs

seems to have resulted from some such process (Glob, 1971). The stomach contents of these bodies have been preserved well enough to permit

identification of the various grains that constituted the last meals, so that

digestive processes must have ceased quite soon after death. It seems unlikely that tannins would diffuse rapidly enough to account for this, and

it has been suggested that the preservation of the stomach contents results

from the action of hydrogen sulfide.

I . Sulfur Reducing Organism

Hydrogen sulfide is formed in peat bogs, etc., as a product of putrefaction. Bacteria of the genus Clustridium are chiefly responsible for the

anaerobic decomposition of protein, but another group of bacteria is a

GENESIS AND MANAGEMENT OF ACID SULFATE SOILS

269

much more important source of hydrogen sulfide in anaerobic soils. These

are the dissimilatory sulfate-reducing bacteria, an exclusive property of

which is the utilization of sulfate in the same sense that higher organisms

use oxygen-i.e., sulfate acts as the terminal electron acceptor for their

respiratory processes. Postgate ( 1959 ) suggested the term assimilatory sulfate reduction to describe the production of sulfur-containing organic cell

constituents, and dissimilatory sulfate reduction for the relatively much

more extensive energy-yielding process. The process can be formalized as:

2CHa.CHOH.COOH

+ SO

1-

+ OCH3.COOH

+ 2C02 + 2H20 + S*-

Stoichiometric amounts of sulfide are formed, and if adequate sulfate is

available, as in marine or estuarine environments, much hydrogen sulfide

is formed at the expense of relatively little organic matter-as much sulfide

would be formed by sulfate-reducing bacteria from 1 mole of lactic acid

as would be provided by several kilograms of putrefying protein.

Beijerinck described the original type species, Desulfovibrio desulfuricans, in 1895. Campbell and Postgate (1965) distinguished two genera,

In biological sulfate reduction experiments, in which the redox potential

was controlled automatically, Connell and Patrick (1968) found that sulfate became unstable at about -150 mV. Under their conditions, the bacterial reduction of sulfate was confined to the pH range 6.5-8.5, which

is a less acid lower limit than usual; pH 5 seems to be about the lowest

value at which anaerobic sulfate reducers are active (Bloomfield,

unpublished).

Hydrogen sulfide diffuses readily, and, unless it is immobilized as an insoluble sulfide, it tends to enlarge the anaerobic zone and extend the environment favorable to the development of sulfate-reducing bacteria.

Sulfate reducers have important effects in causing the precipitation of

metal sulfides, notably of iron, in causing pollution of waters, etc., and

they are responsible for the corrosion of steel buried in certain anaerobic

at the fringe of the anaerobic zone, colorless sulfide oxidizers produce elemental sulfur which is oxidized to sulfate by Thiobacilli, which are discussed later. The cycle can continue indefinitely provided minor elements,

but befork extraction of the sulfur the samples were treated with dilute acid

to decompose monosulfide, so that if acid-soluble ferric compounds were

present at least some of the sulfur could have been formed by oxidation

of hydrogen sulfide by Fe3+during the preparation of the samples. As elementary sulfur is reduced by D. desulfuricans, the formation of iron disulfide in anaerobic sediments must be the net result of competition between ferrous sulfide and sulfate-reducing bacterias for sulfur, as it is

formed by the reaction of hydrogen sulfide with ferric iron; it is to be expected that elementary sulfur would have only a transient existence in an

anaerobic sediment.

GENESIS AND MANAGEMENT OF ACID SULFATE SOILS

273

D. ENVIRONMENTAL

FACTORS

AND SULFIDE

FORMATION

Essential for the formation of sulfides is a supply of sulfates and organic

matter, so coastal and deltaic areas, often very important agriculturally,

provide optimum conditions for formation of sulfides.

1 . Physiography

Knowledge of the physiological conditions for formation of sulfide-bearing muds is useful for understanding their genesis, predicting their location

and mapping their boundaries. Recent coastal deposits cover very large

areas, particularly in the tropics; an example is the west coast of Malaysia

where the deposits may be up to 40 miles wide and 450 feet deep (Carter,

1959). Deltas form at the mouths of all rivers, but those in the tropics

originate from much larger rivers, are flooded to much greater depth at

certain times, and are desiccated more intensely at others.

Fosberg ( 1964) has classified the main physiographic features of deltas

into water (distributaries, delta channels and tidal channels, lakes and

tree species as primary colonizers, and mangrove, a term used to cover

both the ecological group of species on tidal lands of the tropics and the

plant communities that include these species (Richards, 1952), is primarily

involved. There are several families of mangrove, the more important being

the Rhizophoraceae, the Lythraceae, and the Verbenaceae. Rhizophora

mucronate and R . conjugata occur in the Malaysian area and eastern and

southern Africa (Watson, 1928; Dale, 1939; Macnae and Kalk, 1962).

Rhizophora apiculata and R . stylosa are found in Queensland (Macnae,

1966). Around the Atlantic shores the three main species are Rhizophora

mangle, R . racemosa, and R . harrisonii (Keay, 1953; Davis, 1940). Various species of the genus Avicennia occur in the Verbenaceae family. Avicennia germinans occurs in West Africa, A . oficinalis, A . intermedia, and

A . alba in Malaysia, A . marina in Australia, eastern and southern Africa,

and A . nitida in Florida.

In this discussion our interest lies in the relationships, mostly indirect,

between vegetation type and sulfide levels in the mud. Unfortunately

studies on the colonizing vegetation seldom include any information on

the muds, except perhaps on the salinity, but sulfide contents are seldom

recorded. Reeds (Phragmites sp.) are usually related to sulfides in temperate area muds, but these are not primary colonizers of coastal areas.

This suggests that, in these areas, excess sulfides are formed when the soils

are under intermittent flooding by saline waters, i.e., brackish water

swamps. Work in West Africa (Tomlinson, 1957) appears to have first

drawn attention to the relationships between mangrove species and excess

sulfide formation; he found that areas presently or formerly under

Rhizophora racemosa developed much acidity on drainage, whereas soils

from areas of Avicennia did not. He attributed this to differences in the

rooting habits of the species; stilt roots of one Rhizophora tree may cover

an area of 6 m diameter (Watson, 1928). In the soil these stilt roots are

covered with root hairs, the major source of the peaty material which gradually builds up under this vegetation. This peaty layer may be several feet

thick in West Africa, and Rosevear (1947) states that it can be cut and

burned as fuel. Such thick peat deposits have not been reported from

Malaysia, but in Thailand van der Kevie (1973) states that the vegetation

over broad areas of sulfide muds has been swamp forest, “probably

Rhizophora.” The reasons for the different amounts of fibrous peat formed

276

C. BLOOMFIELD AND J. K. COULTER

under Rhizophora in various parts of the world may lie in the reaction

of the tree to ecological factors. Watson (1928) in Malaya, and Davis

(1940) in Florida, recorded that trees growing in shallow soils or areas

subject to very deep inundation have the greatest mass of roots, so that

large root masses would be expected in some areas of West Africa, because

of the great changes in water level that occur under tidal and fresh water

flooding.

By contrast Avicennia sp. produce a shallow widespread root system,

with small pneumatophores protruding from the surface. In West Africa,

soils under this species are not usually fibrous unless previously covered

with Rhizophora. On the other hand, Davis (1940) reports that in Florida

deep peats are formed from the remains of both Rhizophora and Avicennia

seem to be the major type of vegetation in large areas of Vietnam (The

Plain de Joncs) . However, these species, although useful indicators, are

not specific for acid sulfate soils, but they are indicative of generally

adverse soil conditions.

As a conclusion, certain vegetation consociations are indicative of potential or acid sulfate soils, but none are specific; the closeness of the relationships obviously varies from one environment to another, so that extrapolation of relationships that exist, say in West Africa, to East Asia is

unreliable.

3 . Climate

Acid sulfate soils occur in a wide variety of climates, but the largest

areas are in the humid and monsoonal zones of the tropics, and in the

moist temperate climates. The temperature obviously influences the

amount and type of vegetation; although more organic matter is present

in sediments of temperate areas at the time of deposition, the continuous

growth of vegetation in the tropics can add larger quantities to the deposits.

Rainfall distribution affects the behavior of sulfidic muds after deposition; without a marked dry season they may remain in a waterlogged and